I am sorry you had this happen. I would offer to buy the lady some coffee and maybe offer to work through the problems. You could maybe suggest a discount, and together take out the first stitching. Unstitching is a hard lesson to learn, but we have all had to learn it. I would handle it as kindly and firmly as possible. If she works in the LQS she will want happy customers.
I once had a cover built on my patio, and had to be out of town while the work was done. When I came home, I had a good long cry, then called the contractor over for coffee. I calmly explained that "he could not possibly be proud of this job", and "here were pictures for him so he can see the errors that were made during construction". Worked like a charm. I was so pleased by the redo (He had hired a kid to help the first time), that I typed him a letter of reference. I still have a beautiful patio cover. Grandma always said 'you catch more flies with honey than with vinegar' Good luck with this.